The present invention relates generally to an improved overtemperature or fire detection cable and, more particularly, to an overtemperature or fire detection cable having an inner spring wire conductor maintained in substantial coaxial or spaced apart relation with a rigid, electrically conductive outer sheath by a thermoplastic dielectric material.
Prior art fire detection cables have commonly employed two wires wrapped or twisted about each other and electrically separated by their respective dielectric coverings which soften at a predetermined temperature. In such arrangements, a bias force is provided to bring the two wires into electrical contact when the dielectric material softens. This bias force is typically provided by twisting two flexible wires of approximately equal size together and then providing a covering for maintaining the bias force, or by wrapping a flexible outer wire around a larger inner wire.
However, such prior art fire detection cables are particularly susceptible to giving false indications when used in chemically hostile environments where vapors from chemicals such as toluene, cleaning agents and the like attack the dielectric cover and thus allow the wires to come into electrical contact. Also, the operating environment of such fire detection cables is often such that physical contact by heavy machinery and human operators cannot be avoided. This is especially true in off-road motor vehicle applications, in open surface mining operations and in other construction operations. Great difficulty has been experienced in protecting prior art twisted wire arrangements from physical injury, such as that caused by abrasion or compression, which tends not only to alter the amount of twist between the conductors, but which also deforms one or both of the insulating coverings, which coverings are relied upon to melt or otherwise soften in a predetermined and precise manner.
Further, a significant impairment of prior art fire detection cables has been experienced during installation when the cable is subjected to a misplaced hammer blow or accidental contact with a wrench during its attachment to a support member. Such unintentional contact may destroy or otherwise weaken the electrical insulation between conductor wires, as well as alter the amount of twist therebetween. Careful handling and securement is especially critical when installing such prior art fire detection cables which rely upon an accurate and uniform amount of twisting between the wire pair. Changes in the bias force between the wires are also likely to occur if the twisted wires are allowed to sag, for example, when the wires are suspended between two or more support points. Because the application of these cable arrangements is limited, in that they tend to sag when suspended between two points and cannot of themselves withstand pedestrian or light vehicular traffic, a costly conduit system may be required.
Moreover, even when properly installed and maintained, such fire detection cables report only the occurrence of a fire or an overtemperature condition and are not suitable for determining the exact location of the overtemperature condition. Also, in the instance of such condition, the fire can actually spread along the fire detection cable to other locations if the outer fabric covering of the cable ignites.